I'd take Lawler. Leon legitimately beat Usman but I'm not going to forget that his road to the title was not exactly filled with the best names or most convincing performances and he hasn't exactly been active since then to prove that he is suddenly leaps and bounds better.
Considering Usman's age and wear and tear, is it possible that he was on the way out and Leon was in the right place at the right time? Definitely. Does a lukewarm win over Colby Covington aka one of the most overrated fighters in MMA history do much to add to Leon's credibility? IMO no.
Prime Lawler was stuffing takedowns from Rory, Hendricks, Koscheck, etc. and could take an insane amount of damage. I mean look at the leg kicks he was eating from Melvin before getting the KO win.
It'd be a good fight and Leon could absolutely win but I think in a 5 round fight, Leon would get clipped eventually and Lawler is a far scarier person to have swarming you than a half retired Nate Diaz. Lawler via TKO